[bug 914122] Don't use letter-spacing on grid icons, as it results in poor glyph spacing

See bug 914122 comment 40. Using negative letter-spacing gives poor-looking text, because the glyph positions are rounded to whole pixels and therefore fine adjustments to the individual positions cannot be accurately realized; instead, we get a few abrupt, ugly whole-pixel shifts at "random" places within the string.
Merge pull request #13714 from jfkthame/patch-2

Bug 914122: Don't use letter-spacing on grid icons, as it results in po...